How to add to the PYTHONPATH in Windows, so it finds my modules/packages?

Via Windows: setting environment variables

The role of Windows environment variables is to facilitate the smooth operation of applications. In Python, PYTHONPATH operates like a roadmap, guiding the interpreter to modules/packages locations.

Employing system GUI

Windows provides a GUI method to set environment variables in a user-friendly manner:

1. Open System Properties (Right-click Computer/This PC → Properties → Advanced system settings).
2. Select Environment Variables...
3. Under System variables, hit New... to create PYTHONPATH, or choose it and press Edit... to amend.

Utilizing command prompt

For those who prefer hands-on experience or automation, command line comes in handy:

setx PYTHONPATH C:\Python27\Lib;C:\other\module\dirs
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Include your installation directory (like C:\Python27\Lib) and add any crucial directories separating by semicolons. It's like you're showing Python the VIP lounge.

Juggling multiple Python versions

The issue of multiple Python versions may arise. It's like having multiple favorite flavors of ice cream. Luckily, we have the solution. Administer these by setting a variable known as PY_HOME:

setx PY_HOME C:\Python27
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Later, incorporate %PY_HOME% in your system's path:

setx PATH "%PATH%;%PY_HOME%"
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A disciplined PATH is the key to Pythonic harmony. This technique allows for efficient version control.

Sys.path manipulation

In circumstances where you wish to embed this setup in a Python script, sys.path can be fine-tuned:

import sys
sys.path.append(r"C:\My_Projects")  # Keep adding as many directories like toppings on a pizza!
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Remember to employ raw strings (prefix with r) to avoid obstacles with escape characters in Windows paths.

Checking your work. Is the oven preheated?

Post setting up system variables, whether via GUI or command line, a system or command prompt restart is imperative to ensure the changes come into effect. Validate your Python's accessibility by inputting python in the command prompt. Keep a check for any hiccups or discrepancies in your PYTHONPATH and module names. You don't want a typo to spoil the party!

When Murphy's Law strikes

Troubleshooting is intrinsic to any setup. Let's combat common setbacks:

Validate your environment

Check your environment variables:

echo %PYTHONPATH%
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Ensure directories enlisted in PYTHONPATH are bona fide and reachable.

The saga of the missing module

In the event of No module named coltrane, cross-verify that the directory in PYTHONPATH incorporates the missing module, and the module is aptly named.

Shifting Python versions swiftly

With the PY_HOME technique discussed, swap between installed Python versions by updating the PY_HOME variable.

setx PY_HOME C:\Python39
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Follow it up with a system or Command Prompt restart to refresh the audience.

Authenticating Python is operational

Upon setting PY_HOME and adjusting PATH, confirm python is operational from any directory in your command prompt:

python --version  # Python 3.9.5: "My anaconda don't want none unless you got (Python 3.9.5) hun!"
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